A magnetic resonance (MR) imaging system provides an image of at least a portion of a patient, animal or object in an imaging volume based on detected radio frequency (RF) signals from precessing nuclear magnetic moments. In general operation, a main magnet produces a static magnetic field over the imaging volume and gradient coils within the MR imaging system are used to quickly switch to effect magnetic gradients along mutually orthogonal x, y, z coordinates in the static magnetic field during selected portions of an MR imaging data acquisition cycle. There is also an RF coil that produces RF magnetic field pulses perpendicular to the static magnetic field, within the imaging volume to excite the nuclei. The nuclei are thereby excited to precess about an axis at a resonant RF frequency. As the precession occurs into the transverse plane, the transverse component of magnetization is magnetically coupled to some external circuitry, typically a receiver. The transmitter and receiver coupling mechanisms are typically referred to as RF coils.
For a typical multi-nuclei scanner, the RF coils and corresponding amplifiers receive excitation pulses from a respective transmitter that is configured to generate a plurality of excitation pulses in a spectrum around the resonance frequency of a particular isotope. During excitation, pulses from an associated amplifier energize the respective RF coil, which is frequency-tuned to the particular isotope of interest within the subject. During data readout, a switch connects the RF coil to a receiver so that MR signals generated from precessing nuclei within the subject are received by the RF coil and conveyed to the receiver. The acquired MR signals are processed to produce one or more images of the subject.
Natural radiation damping (RD) occurs when the nuclear magnetic resonance (NMR) signal induced in a receiver coil is strong enough to generate a significant RF magnetic field acting back on the spins. According to Lenz's law, the RD field acts in a way to oppose its original cause. In that sense, it is understood as a self-regulating flip-back pulse causing the transverse magnetization to return to equilibrium more rapidly than it otherwise would, such as described in Bloembergen N, Pound R V; “Radiation Damping in Magnetic Resonance Experiments”, Physical Review 1954; 95(1):8-12. The return to equilibrium occurs at a characteristic rate distinct from tissue-specific relaxation times. The RD phenomenon has been demonstrated for frequency-dependent contrast. This is further explained in Huang S, Chung A L, Y.; “Visualizing Feedback-Enhanced Contrast in Magnetic Resonance Imaging”; Concepts in Magnetic Resonance Part A 2007; 30A(6):378-393.
For clinical MRI scanners operating at 1.5 or 3 T, the coil quality-factor (Q) and the filling-factor (η) of standard RF coils are insufficient to induce a significant intrinsic RD field. Typically, signals are sufficiently strong to produce RD only in very high quality-factor (Q) coils with high filling-factors (η), such as in high-resolution NMR spectrometers. Recently, actively controlled amplified radiation damping (ARD) feedback loops have been introduced into the transmit-receive signal path as a means to either cancel or amplify the RD field, in the context of high-resolution NMR, or MRI systems.
Certain applications for canceling the RD field are described in Broekaert P, Jeener J.; “Suppression of radiation damping in NMR in liquids by active electronic feedback”; Journal of Magnetic Resonance, 1995; 113:60-64 and Louis-Joseph A A, D., Lallemand J Y.; “Neutralization of radiation damping by selective feedback on a 400 MHz NMR spectrometer”; Journal of Biomolecular NMR 1995; 5:212-216.
Certain applications for amplifying the RD field are detailed in Abergel D, Louis-Joseph A L, J. Y, “Amplification of radiation damping in a 600 MHz NMR spectrometer”; Journal of Biomolecular NMR 1996; 8:15-22; and Huang S Y, Witzel T, Wald L L. “Accelerated radiation damping for increased spin equilibrium (ARISE): a new method for controlling the recovery of longitudinal magnetization”; Magnetic Resonance Medicine, 2008; 60(5):1112-1121.
The signal-to-noise ratio (SNR) efficiency in MR experiments is intrinsically limited by longitudinal and transverse spin relaxation mechanisms. This becomes particularly limiting for short-repetition-time (TR) pulse sequences, where incomplete T1-relaxation results in a steady-state signal that is only a small fraction of the available thermal equilibrium magnetization. In comparison, fast recovery and steady-state free-precision (SSFP) are known to be among the most SNR efficient pulse sequences, as explained in Bernstein M A, King K F, Zhou X J; “Handbook of MRI Pulse Sequences”; Elsevier Academic Press 2004. Fast recovery sequences restore remaining transverse magnetization by rotating it back to the longitudinal direction. SSFP uses signal refocusing and RF phase cycling to recycle the magnetization efficiently with relatively high flip angles and fast repetition. Both fast recovery and SSFP methods are susceptible to various imperfections (i.e. off-resonance effects, flip-angle miss-calibration, transient effects, undesired refocusing echoes, etc.) and are therefore demanding in terms of robust implementation.
While there have been a variety of efforts to implement ARD feedback schemes, there continue to be unfavorable characteristics in the state of the art that remain unresolved. In particular, previously presented ARD feedback schemes are generally limited in terms of flexibility and their ability to achieve adequate feedback gain.